( 171 ) 



XVI. — On the Rejiresentative Rslationships of the Fixed and Free Tunicata, re- 

 garded as two Subclasses of equivalent value; with some General Remarks on their 

 Morphology. By John Denis Macdonald, R.N., F.R.S, Surgeon of PI.M.S. 

 " Icarus." Communicated by Professor Maclagan. (Plate IX.) 



(Read 15th December 1862.) 



My first precise views of the structure of the Tunicata were formed by the 

 perusal of Mr Huxley's masterly papers dealing with the anatomy of Salpa, Py- 

 rosoma, Doliolum, and A^jjyendicularia, in the Phil. Trans, for 1851, Part 11. ; and 

 I have since had abundant opportunity of verifying all the important facts, made 

 known by that original observer, in the papers to which I have alluded. 



Having thus acknowledged my guide in this field of research, I can scarcely 

 claim much originality as relates to pure anatomy ; but I hope that the method 

 here adopted, in opening up this interesting subject, will be found in keeping with 

 nature, as it is the result of much study and practical investigation. 



I must first beg the question, and next endeavour to support it, that the class 

 Tunicata may be conveniently divided into two subclasses — viz., the fixed or 

 stationary, and the free or locomotive. The latter, from their habit of life, are 

 also commonly denominated Pelagic; while the former, by general consent, chiefly 

 following the suggestions of M. Milne-Edwards, have been divided into the 

 Simple, the Social, and the Compound, as given in the following table : — 



Tunicata. 

 I. Fixed or stationary. 



1. Solitary, or simply segregate. 



a. Sessile (recumbent or erect), or pedunculate, .... Simple. 



2. Organically blended in communities. 



a. Sessile or pedunculated on a common axis, .... Social. 



b. Immersed in a common test substance, ..... Compound. 

 II. Free or locomotive, ......... Pelagic. 



Of the fixed Tunicata, it would appear that Pelonaia and Chelyosoma* pre- 

 sent characters which at once distinguish them from all the other members of 

 the subclass. Thus, in the genera named, the branchial membrane seems to 

 be closely adherent to the subjacent textures, without forming a distinct sac, 



* We ai'e much in want of more accurate information respecting these genera. Mr Huxley 

 remarks, loc. cit. 588, " In Pelonaia, the hypopharyngeal band has disappeared. It is a Salpa in 

 which tlie oral and cloacal orifices have approximated, while the ' gill' lias become obliterated;" and 

 in a note at the bottom of the page he says, " Chelyoftoma would appear to resemble Pelonaia in the 

 absence of any distinct branchial sac; but Eschricht's figures are not very clear." 



VOL. XXIII. PART II. • 3 B 



